The present disclosure relates generally to systems and methods for well logging. In particular, the present invention is an apparatus and method for the imaging of subsurface formations.
Modern oil field operations use information related to the parameters and conditions encountered downhole to aid in drilling decisions. For example, measurements may be made using tools located within a borehole. Such information typically includes characteristics of the earth formations traversed by the borehole, and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging and “logging while drilling” (LWD).
In wireline logging, a sonde is lowered into the borehole after a portion of a well has been drilled. The sonde hangs at the end of a wireline cable that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated.
Among the available wireline and LWD tools for evaluating geological formations are various resistivity logging tools including, galvanic or laterolog tools. Such tools can use electrodes to inject electrical currents into surrounding earth formation for taking resistivity measurements. An apparent resistivity (or conductivity) of a formation may be evaluated such as by injecting the electrical currents from a location within the borehole into a portion of the formation, and conductively measuring a resulting voltage induced by the current. Such resistivity information may provide a general indication of formation composition or geometry, including providing indicia of invasion or hydrocarbon presence.
For example, it is often desirable to construct an image of the borehole wall. Among other things, such images reveal the fine-scale structure of the penetrated formations. The structure includes stratifications such as shale/sand sequences, fractures, and non-homogeneities caused by irregular cementation and variations in pore size. Orientations of fractures and strata can also be identified.